1. Field of the Invention
The present invention relates to a method for preparing dialkylnaphthalenes from alkenylbenzene compounds as a starting material. More particularly, the present invention relates to a method for preparing dialkylnaphthalenes by cyclizing and dehydrogenating alkenylbenzene compounds in one step.
2. Description of the Prior Art
Naphthalenedicarboxylic acids obtained by the oxidation of dialkylnaphthalenes are useful as a dibasic acid component for preparing polyesters. In particular, polyesters containing naphthalene-2,6-dicarboxylic acid as a dibasic acid component are well known to have excellent properties.
Roughly grouping, two methods have hitherto been known for the preparing of dialkylnaphthalenes. More specifically, there have been proposed a method which derives dialkylnaphthalene from naphthalene compounds, and a method which derives them from benzene compounds.
Examples of the method which uses naphthalene compounds as a starting material include methods in which naphthalenes are methylated (Japanese Patent Applications Laid-Open Nos. 45536/1985 and 172937/1985), a method in which naphthalenes are ethylated (Japanese Patent Applications Laid-Open Nos. 6953/1976 and 500179/1991 corresponding to U.S. Pat. No. 4,873,386), methods in which naphthalenes are propylated (WO90/03961, and Japanese Patent Applications Laid-Open Nos. 9942/1989 and 9943/1989), methods in which naphthalenes are acylated (Japanese Patent Applications Laid-Open Nos. 53937/1987, 188343/1985 and 135756/1979), and so on. These methods not only suffer from problems of impurities contained in the naphthalene compounds used as a raw material, and supply of purified naphthalenes, but also are less competitive from the viewpoint of cost.
On the other hand, the following methods have been proposed that derive dialkylnaphthalenes from alkylbenzenes. One is a method in which a starting material obtained by acylating alkylbenzene in the presence of a HF/BF.sub.3 catalyst is subjected to a hydrogenating step, a dehydrating step and a cyclizing and dehydrogenating step to prepare a dialkylnaphthalene (Japanese Patent Applications Laid-Open Nos. 96540/1990 and 167237/1990). This method cannot be said to be advantageous industrially because of long procedure and use of a HF/BF.sub.3 catalyst. Further, as the cyclizing and dehydrogenating catalyst, there have been proposed metal oxide catalysts such as alumina-chromia and iron oxide catalysts, or those catalysts containing precious metals such as platinum and palladium carried on alumina or activated carbon. There is cited Cr.sub.2 O.sub.3 --K.sub.2 O--Al.sub.2 O.sub.3 as an example, which gives a very low selectivity for the objective compound.
Whereas a method has been known in which dimethylnaphthalene is obtained from a tolylpentene compound as a starting material by cyclization-dehydrogenation in one step. For example, U.S. Pat. No. 3,931,348 discloses use of a catalyst containing a combination of an alkali metal and chromia-alumina, a catalyst containing rhenium oxide, an alkali metal, and an alkaline earth metal carried on alumina, etc., as a cyclization-dehydrogenation catalyst. Their yields are too low to be industrially acceptable.
U.S. Pat. No. 3,244,758 suggests a method in which cyclization and dehydrogenation are performed simultaneously using platinum-alumina or platinum-silica alumina as a catalyst, without showing concrete data of actual reactions. Further, Japanese Patent Publication No. 1036/1975 describes a method in which the aforementioned one-step reaction is performed in a gas phase using a palladium catalyst. This method suffers from vigorous decomposition of the starting material and yield of dimethylnaphthalene, the objective compound, is low. Further, Japanese Patent Publication No. 1037/1975 described a method in which the aforementioned one-step reaction in a liquid phase using a silica alumina catalyst carrying thereon a metal such as platinum, palladium, rhodium or rhenium. This method, too, gives a low conversion of the starting material, and suffers from decomposition of the starting material, resulting in that it is unsuitable as an industrial process.
Also, a method has been proposed in which dimethylnaphthalene is prepared from xylenes and butadiene (Japanese Patent Publication Nos. 12430/1975 and 48494/1981). This method uses tolylpentenes obtained by alkenylating xylene with butadiene as a starting material and prepares dimethylnaphthalene compounds, the objective compounds, via a cyclizing step, a dehydrogenating step and a isomerizing step.
While this method is featured in that each reaction thereof has a very good selectivity and proceeds in a high yield, it is disadvantageous in that not only it involves a long series of steps but also it requires a complicated apparatus since the cyclizing step, which is an exothermic reaction, and the dehydrogenating step, which is an endothermic reaction, are independent of each other. Conventional proposals for carrying out these reactions in one step reaction have been unacceptable industrially as described above. In particular, alkenylbenzene compounds which are a starting material are susceptible to hydrogenation reaction, a side reaction, with hydrogen generated or carrier hydrogen and converted to alkylbenzene compounds. As a result, cyclization reaction is difficult to proceed, thus giving poor selectivity and yield.